&DRIFT: ARRIVAL-TIME-DISTRIBUTION


electron

  The electrons on which you trigger. These are usually small
  numbers, around 5, but you are free to choose larger values.
  You may list several electrons, up to a compilation dependent
  limit (currently set to 10).

  In the rest of the descriptions, these electrons are referred
  to as the 'selected electrons'.

  If a value is negative, then the electrons will be counted
  from the back, the last electron corresponds to a value of 0.
  For the 4 last electrons, you can also type LAST, ONE-BUT-LAST,
  TWO-BUT-LAST and THREE-BUT-LAST.

  Default: initially, only the 25th electron is selected. The
  set of selected is remembered as default for the next call.

threshold

  A probability between 0 and 1 (both excluded). Apart from
  the mean arrival time of the selected electron, Garfield
  outputs also the time at which the selected electron has a
  given probability to have arrived.

  Default: initially 0.5, the median of the distribution. The
  value set is kept as default for the next call.

AUTOSCALE

  Whilst the program can compute reasonable bounds for the
  overall arrival time distribution histogram, it can not do
  so for the histogram of the selected electron.

  Unless an explicit time window is specified, Garfield will
  therefore take the computed time range for the all-electrons
  arrival time histogram, and it will book an histogram with
  automatic range calculation for the arrival time distribution
  of the selected electron. Automatic range calculation means
  that the first 100 entries are used to compute a suitable
  range.

  If you wish the selected electron arrival time distribution
  histogram to have the same range as the histogram with the
  distribution for all electrons, then you should specify the
  NOAUTOSCALE-TIME-WINDOW option. Keep in mind, if you do so,
  that the accuracy of the median arrival time of the selected
  electron can be reduced because the arrival time distribution
  is likely to cover only a small fraction of the range of the
  histogram.

  If you wish to specify a time window yourself, then use the
  TIME-WINDOW keyword.

  Default: automatic scaling is on by default.

TIME-WINDOW

  Specifies the time window of all arrival time histograms, both
  of the selected electrons and of all electrons.

  This option is most useful if used in conjunction with the
  KEEP-HISTOGRAMS option when you wish to study the histograms
  in more detail.

  Default: by default the time range of the selected electron
  histogram is computed from the first 100 entries of this
  histogram, whereas the range of the all-electron histogram
  is set to the time range for the track under study.

X-RANGE

  The part of the AREA to be covered by the tracks.

  Default: the entire x-range of the AREA.

x_step

  The x-distance between 2 tracks.

  Default: about 5 % of the horizontal scale.

Y-RANGE

  The vertical extent of the tracks. This parameter is less
  trivial than it might appear: if the range does not cover
  the full acceptance boundaries of the wire, the program
  will give wrong results for late electrons. If the range
  is chosen too large, the waste of CPU time is considerable
  and also the accuracy may suffer if the LINES parameter
  does not compensate.

  Default: the entire y-range of the AREA.

DIFFUSION

  By default, diffusion is included in the arrival time
  calculations. If you wish to study the impact diffusion
  has on the distributions, then it may be convenient to
  be able to switch diffusion off. This can be achieved
  by selecting the NODIFFUSION option.

ATTACHMENT

  Depending on the presence of attachment data or not,
  attachment is or is not included. If attachment data
  is present, but if you do not want the effect to be
  taken into account, then you can specify NOATTACHMENT.

lines

  Each track is first 'prepared': from regularly spaced points
  on the track, electrons are drifted and the drift times and
  integrated diffusion coefficients are stored for interpolation
  when doing the Monte-Carlo cycles. Where the drift time
  changes most between 2 points, additional drift lines are
  calculated.

  The parameter lines is the total amount of drift lines that
  are calculated this way: 75 % in the first step and 25 % in
  the second.

  Default: the LINES parameter from the &DRIFT section.

phi

  The angle of the tracks with respect to the y-axis.

  Default: initially 0 degrees. The angle that you set is
  kept as default for the next call.

bins

  The number of bins in the histograms used to compute the
  time at which the selected histogram has a given probability
  to have arrived - the mean arrival time is computed directly
  without histogram.

  Default: initially half of MXLIST, usually 100-500 bins.
  The value that you set is kept as default for the next
  call.

KEEP-HISTOGRAMS

  This option can be used to store the arrival time histograms
  for later study. The histograms are made accessible via global
  variable names that are displayed while the command is carried
  out (SEL_n, and ALL_n with n a counter).

  A common application of this option seems to be the Gaussian
  fit of the arrival time histograms. This can be done using the
  FIT_GAUSSIAN procedure, or by a user program after writing the
  histograms to disk with the WRITE_HISTOGRAM procedure.

  If (1) the instruction is placed in the body of a DO loop and
  (2) these global variables are used in arithmetic or in IF
  conditions, then you have to declare the global variables before
  the DO loop by assigning them an (arbitrary value). Declaring is
  not required if the variables are only used outside a DO loop,
  e.g. in SAY statements or in procedure calls.

  Default: histograms are not kept. The setting is kept across calls
  of ARRIVAL.

KEEP-RESULTS

  If this option is selected, then the x(t) relations that have been
  obtained by ARRIVAL, will be stored as a set of matrices and numbers:

  -----------------------------------------------------------------
  Global                 Description                      Type
  -----------------------------------------------------------------
  X_wire                 x-coordinates                    Matrix
  MEAN_wire              mean time all electrons          Matrix
  MEDIAN_wire            median time all electrons        Matrix
  RMS_wire               RMS time all electrons           Matrix
  MEANelectron_wire      mean time selected electron      Matrix
  MEDIANelectron_wire    median time selected electron    Matrix
  RMSelectron_wire       RMS time selected electron       Matrix
  E_electron             sequence # selected electron     Number
  -----------------------------------------------------------------

  The string "wire" in the global variable name is equal to the wire
  number of the wire for which the x(t) relation is calculated.

  The string "electron" is an index that runs from 1 to the number
  of selected electrons. E_electron shows which electron a given
  index corresponds to. This can be zero or a negative number: 0 means
  the last electron, -1 the one but last etc.

  If (1) the instruction is placed in the body of a DO loop and
  (2) these global variables are used in arithmetic or in IF
  conditions, then you have to declare the global variables before
  the DO loop by assigning them an (arbitrary value). Declaring is
  not required if the variables are only used outside a DO loop,
  e.g. in SAY statements or in procedure calls.

  This option is not on by default, but its setting is remembered
  from one call to the next.

loops

  The number of Monte Carlo loops over track generation. These
  loops consume relatively little CPU time since the drift times,
  diffusion coefficients etc are all taken from interpolation
  tables.

  It seems therefore tempting to select a large value for this
  parameter. When saving the distributions (KEEP-HISTOGRAM) and
  then using them in a fit, this may indeed be a good approach.
  However, when extracting information from the MC process with
  means and RMS, the accuracy tends to degrade for large values
  of loop, mainly because of single entries with very deviating
  values.

  When specifying SINGLE-CLUSTER, only one cluster is generated
  per track, as a result the default value will be low and you
  are advised to choose a larger value in this case.

  Default: initially 1000. Each ARRIVAL statement has as default
  the value set in the previous invocation.

order

  The time by which the selected electron has a certain
  probability to have arrived, is obtained by reverse polynomial
  interpolation in the cumulative timing histogram. Using this
  keyword, you can change the order of the polynomial.

  Default: 2, parabolic interpolation. The setting will be
  remembered from one call to the next.

dsname

  The name of the dataset you wish to write your results to.
  If you don't specify such a dataset name, you will only
  receive plots and no numeric output.

  Default: By default no dataset is written.

member

  The name to be assigned to the member.

  Default: none.

remark

  Garfield uses by default a string specifying the wire number
  as remark for the member header. You may override this.

plot_options

  Garfield can be asked to output the following graphs for
  each track:

  -------------------------------------------------------------------
  Option                        Plot produced
  -------------------------------------------------------------------
  PLOT-EACH-X-OVERALL           The arrival time distribution of all
                                electrons from the track that crosses
                                a line parallel to the x-axis at the
                                value of x currently looked at.
  PLOT-EACH-X-SELECTED-ELECTRON The arrival time distributions for
                                the selected electrons from the track
                                that crosses a line parallel to the
                                x-axis at the value of x currently
                                looked at.
  -------------------------------------------------------------------

  Both plots are by default not made.

  In addition, plots can be requested that summarise the results of
  the computation for each wire, as function of x. The PLOT-OVERVIEW
  option will ensure that the following 3 graphs are made:

  1. The mean drift time of all electrons and the mean drift time
     of the selected electrons.

     This graph is a more refined version of the x(t) relation as
     calculated by the XT-PLOT instruction.

  2. The drift time by which any electron has a probability of
     'threshold' to have arrived and the drift time by which the
     selected electrons have this probability to have arrived.

     Since the 'threshold' is usually set to 50 %, this probability
     is in practice equivalent to the median.

  3. The spread of the arrival time distribution of all electrons,
     and the spread of the arrival time distributions of the selected
     electrons.

     The spread in shown in this graph is the standard deviation of
     the arrival time histogram. This measure is sensitive to
     outliers, and should not be used for delicate studies of the
     resolution. The KEEP-HISTOGRAMS option can be used to store
     the histograms, more robust techniques can then be applied
     outside Garfield.

  In all plots, the all-electron graphs are plotted with the
  graphics representation FUNCTION-1, while the graphs for the
  selected electrons are plotted as FUNCTION-2.

  These plots are initially made by default, but the default
  setting of each call is the setting of the previous call.

print_options

  The timing histograms for the selected electrons and for
  all electrons are printed out at your request. In the printout,
  additional information such as the mean and width are provided.
  These quantities are computed from the entries themselves, not
  from the accumulated bin contents. The internal arithmetic is
  done in double precision.

  The setting is remembered from one call to the next.

Keyword index. Formatted on 10/11/98.